EFFECTS OF INACTIVITY ON MYOSIN HEAVY-CHAIN COMPOSITION AND SIZE OF RAT SOLEUS FIBERS

Myosin heavy chain (MHC) and fiber size properties of the adult rat so leus were determined after 4-60 days of complete inactivity, i.e., lum bar spinal cord isolation. Soleus atrophy was rapid and progressive, i .e., 25% and 64% decrease in weight and 33% and 75% decrease in fiber size after 4 and 60 days of inactivity, respectively. Changes in MHC o ccurred at a slower rate than the atrophic response. After 15 days the re was de novo expression of type IIx MHC (similar to 10%). By 60 days , type IIx MHC accounted for 33% of the total MHC content, and 7% of t he fibers contained only type IIx MHC. The relative amount of type I M HC was reduced from 93% in control to 49% after 60 days of inactivity. Therefore, the effects of 60 days of inactivity suggest that during t his time period at least 75% of fiber size and similar to 40% of type I MHC composition of the adult rat soleus can be attributed to activat ion-related events. (C) 1998 John Wiley & Sons, Inc.